Various methods and apparatus that have been devised in the past for fabricating an offshore structure or marine vessel of extended length at an offshore location are exemplified by U.S. Pat. No. 3,407,771 to Takezawa et al., granted Oct. 29, 1968, and U.S. Pat. No. 3,421,469 to Yamada et al., granted Jan. 14, 1969. These prior art methods and apparatus were concerned, however, with fabricating an extended structure at an offshore site which was made up of singular interconnectable structural modules or sections and they are not concerned with or readily adaptable for joining multilegged structures at an offshore site in order to form a unitized elongated structure.